Although charge coupled devices have been used to control the image displayed by a liquid crystal layer, in the prior art the implementation of charge coupled device control of liquid crystal layers has required the use of a nearly intrinsic semiconductor layer and the application of a d.c. voltage across the liquid crystal layer. Such a device is exemplified in U.S. patent application Ser. No. 005,148, filed Jan. 22, 1979 by Grinberg, Waldner and Jenney, entitled "CCD Readout Structure for Display Applications" and assigned to the assignee of the present application. In the Grinberg application, a charge coupled device (CCD) for controlling the image display of a liquid crystal layer includes a nearly intrinsic silicon substrate having a charge coupled device serial register formed in a planar epitaxial silicon layer on the bottom surface of the substrate and a planar liquid crystal layer formed on the top surface of the substrate. Each charge packet stored in the charge coupled device drifts through the intrinsic layer toward a small overlying portion of the liquid crystal layer under the influence of a vertical d.c. electric field so that the electric field, and therefore the optical properties, of the small liquid crystal portion are changed. Thus, using a large plurality of charge packets stored at selected locations in the bottom epitaxial layer, the optical properties of a corresponding plurality of overlying portions of the top liquid crystal layer may be modulated. The liquid crystal layer may be considered to be divided into a plurality of small portions, each small portion corresponding to one of a large plurality of matrix elements of a complete image formed in the liquid crystal layer.
One disadvantage of such a device is that the charge packets may diffuse as they drift through the substrate toward the liquid crystal layer, thus distorting or diffusing the image ultimately displayed by the liquid crystal layer. Therefore, charge packet diffusion in the substrate must be minimized by forming the substrate to be as nearly intrinsic as possible and by forming the substrate to be of a minimum thickness. A related difficulty is that the thin intrinsic substrate must be extremely flat and of a uniform thickness. Otherwise, the response of the liquid crystal to each charge packet may be different depending upon its location, which would introduce spatial nonuniformities into the image displayed by the liquid crystal layer. Another difficulty is that the d.c. electric field supporting the charge packet drift through the intrinsic silicon layer requires a d.c. voltage to be applied to the liquid crystal layer. It is well known that liquid crystals deteriorate faster whenever a d.c. voltage is applied across the liquid crystal.
In summary, the vertical charge packet drift through the intrinsic substrate of the prior art may be characterized as a current source creating a current of charge packets altering the electric field in selected matrix elements of the liquid crystal layer in order to control the displayed image, requiring the use of an intrinsic substrate and the application of a d.c. voltage across the device.